High-yield production of mono- or few-layer transition metal dichalcogenide nanosheets by an electrochemical lithium ion intercalation-based exfoliation method

Liquid Nitrogen Exfoliation and Nanodispersion of WS2 for Thermocatalysts

The efficient and clean exfoliation of single and/or few-layer nanosheets of WS2, a two-dimensional transition metal dichalcogenides, remains a significant challenge. In this study, a simple exfoliation method was proposed to produce ultrathin WS2 nanosheets by combining the liquid nitrogen exfoliation and nanodispersion techniques. This approach efficiently exfoliated WS2 into several layers of nanosheets via rapid temperature changes and mechanical stress without inducing defects or contamination. After five cycles of heating/liquid nitrogen and nanodispersion, the resulting WS2 nanosheets (WS2-5N ND) were confirmed to have been successfully exfoliated into 1-4 layers. When applied as a promoter in a thermocatalyst for the selective catalytic reduction of NOX using NH3, 2V3WS2/Ti (WS2-5N ND) exhibited excellent NOX conversion and N2 selectivity, along with excellent durability even in the presence of SO2. This result was greater than 2V3WS2/Ti (WS2-5N) subjected to only liquid nitrogen exfoliation, proving the importance of the simultaneous action of both methods. This method is expected to be an important contribution to ongoing research on high-performance WS2-based catalysts, thereby opening up potential opportunities for a wide range of applications.

2D Tantalum Disulfide Reduction Strategy Customized Ta2O5/rGO Heterointerface Aerogel Toward Boosting Electromagnetic Wave Absorption and Flame Retardancy

The exceptional and substantial electron affinity, as well as the excellent chemical and thermal stability of transition metal oxides (TMOs), infuse infinite vitality into multifunctional applications, especially in the field of electromagnetic wave (EMW) absorption. Nonetheless, the suboptimal structural mechanical properties and absence of structural regulation continue to hinder the advancement of TMOs-based aerogels. Herein, a novel 2D tantalum disulfide (2H-TaS2) reduction strategy is demonstrated to synthesize Ta2O5/reduced graphene oxide (rGO) heterointerface aerogels with unique characters. As the prerequisite, the defects, interfaces, and configurations of aerogels are regulated by varying the concentration of 2H-TaS2 to ensure the Ta2O5/rGO heterointerface aerogels with appealing EMW absorption properties such as a minimum reflection loss (RLmin) of -61.93 dB and an effective absorption bandwidth (EAB) of 8.54 GHz (7.80-16.34 GHz). This strategy provides valuable insights for designing advanced EMW absorbers. Meanwhile, the aerogel exhibits favorable thermal insulation performance with a value of 36 mW m-1 K-1, outstanding fire resistance capability, and exceptional mechanical energy dissipation performance, making it promising for applications in the aerospace industry and consumer electronics devices.

Lithiation: Advancing Material Synthesis and Structural Engineering for Emerging Applications

Lithiation, a process of inserting lithium ions into a host material, is revolutionizing nanomaterials synthesis and structural engineering as well as enhancing their performance across emerging applications, particularly valuable for large-scale synthesis of high-quality low-dimensional nanomaterials. Through a systematic investigation of the synthetic strategies and structural changes induced by lithiation, this review aims to offer a comprehensive understanding of the development, potential, and challenges associated with this promising approach. First, the basic principles of lithiation/delithiation processes will be introduced. Then, the recent advancements in the lithiation-induced structure changes of nanomaterials, such as morphology tuning, phase transition, defect generation, etc., will be stressed, emphasizing the importance of lithiation in structural modulation of nanomaterials. With the tunable structures induced by the lithiation, the properties and performance in electrochemical, photochemical, electronic devices, bioapplications, etc. will be discussed, followed by outlining the current challenges and perspectives in this research area.

Recent Advances in Engineering of 2D Layered Metal Chalcogenides for Resistive-Type Gas Sensor

2D nanomaterials have triggered widespread attention in sensing applications. Especially for 2D layered metal chalcogenides (LMCs), the unique semiconducting properties and high surface area endow them with great potential for gas sensors. The assembly of 2D LMCs with guest species is an effective functionalization method to produce the synergistic effects of hybridization for greatly enhancing the gas-sensing properties. This review starts with the synthetic techniques, sensing properties, and principles, and then comprehensively compiles the advanced achievements of the pristine 2D LMCs gas sensors. Key advances in the development of the functionalization of 2D LMCs for enhancing gas-sensing properties are categorized according to the spatial architectures. It is systematically discussed in three aspects: surface, lattice, and interlayer, to comprehend the benefits of the functionalized 2D LMCs from surface chemical effect, electronic properties, and structure features. The challenges and outlooks for developing high-performance 2D LMCs-based gas sensors are also proposed.

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Metallic 1T/1T' phase TMD nanosheets with enhanced chemisorption sites for ultrahigh-efficiency lead removal

Two-dimensional (2D) materials, as adsorbents, have garnered great attention in removing heavy metal ions (HMIs) from drinking water due to their extensive exposed adsorption sites. Nevertheless, there remains a paucity of experimental research to remarkably unlock their adsorption capabilities and fully elucidate their adsorption mechanisms. In this work, exceptional lead ion (Pb2+) (a common HMI) removal capacity (up to 758 mg g-1) is achieved using our synthesized metallic 1T/1T' phase 2D transition metal dichalcogenide (TMD, including MoS2, WS2, TaS2, and TiS2) nanosheets, which hold tremendous activated S chemisorption sites. The residual Pb2+ concentration can be reduced from 2 mg L-1 to 2 μg L-1 within 0.5 min, meeting the drinking water standards following World Health Organization guideline (Pb2+ concentrations <10 μg L-1). Atomic-scale characterizations and calculations based on density functional theory unveil that Pb2+ bond to the top positions of transition metal atoms in a single-atom form through the formation of S-Pb bonds. Point-of-use (POU) devices fabricated by our reported metallic phase MoS2 nanosheets exhibit treatment capacity of 55 L-water g-1-adsorbent for feed Pb2+ concentration of 1 mg L-1, which is 1-3 orders of magnitude higher than other 2D materials and commercial activated carbon.

Customization of Manganese Oxide Cathodes via Precise Electrochemical Lithium-Ion Intercalation for Diverse Zinc-Ion Batteries

Manganese oxide-based aqueous zinc-ion batteries (ZIBs) are attractive energy storage devices, owing to their good safety, low cost, and ecofriendly features. However, various critical issues, including poor conductivity, sluggish reaction kinetics, and unstable structure still restrict their further development. Oxygen defect engineering is an effective strategy to improve the electrochemical performance of manganese oxides, but challenging in the accurate regulation of oxygen defects. In this work, an effective and controllable defect engineering strategy-controllable electrochemical lithium-ion intercalation - is proposed to tackle this issue. The incorporation of lithium ions and oxygen defects can promote the conductivity, lattice spacing, and structural stability of Mn2O3 (MO), thus improving its capacity (232.7 mAh g-1), rate performance, and long-term cycling stability (99.0% capacity retention after 3000 cycles). Interestingly, the optimal ratio of intercalated lithium-ion varies at different temperature or mass-loading of MO, which provides the possibility to customize diverse ZIBs to meet different application conditions. In addition, the fabricated ZIBs present good flexibility, superior safety, and admirable adaptability under extreme temperatures (-20-100 °C). This work provides an inspiration on the structural customization of metal oxide nanomaterials for diverse ZIBs, and sheds light on the construction of future portable electronics.

Versatile Method for Preparing Two-Dimensional Metal Dihalides

Ever since the ground-breaking isolation of graphene, numerous two-dimensional (2D) materials have emerged with 2D metal dihalides gaining significant attention due to their intriguing electrical and magnetic properties. In this study, we introduce an innovative approach via anhydrous solvent-induced recrystallization of bulk powders to obtain crystals of metal dihalides (MX2, with M = Cu, Ni, Co and X = Br, Cl, I), which can be exfoliated to 2D flakes. We demonstrate the effectiveness of our method using CuBr2 as an example, which forms large layered crystals. We investigate the structural properties of both the bulk and 2D CuBr2 using X-ray diffraction, along with Raman scattering and optical spectroscopy, revealing its quasi-1D chain structure, which translates to distinct emission and scattering characteristics. Furthermore, microultraviolet photoemission spectroscopy and electronic transport reveal the electronic properties of CuBr2 flakes, including their valence band structure. We extend our methodology to other metal halides and assess the stability of the metal halide flakes in controlled environments. We show that optical contrast can be used to characterize the flake thicknesses for these materials. Our findings demonstrate the versatility and potential applications of the proposed methodology for preparing and studying 2D metal halide flakes.

Solid-state surfactant templating for controlled synthesis of amorphous 2D oxide/oxyhydroxide nanosheets

As a member of 2D family, amorphous 2D nanosheets have received increasing attention due to their unique properties that are distinct from crystalline 2D nanosheets. However, compared with the vast library of crystalline 2D nanosheets, amorphous 2D nanosheets are still infancy due to the lack of an efficient synthetic approach. Here, we present a strategy that yields a library of 10 distinct amorphous 2D metal oxides/oxyhydroxides using solid-state surfactant crystals. A key feature of this process is a stepwise reaction using solid surfactant; the solid-state surfactant crystals have metal ions arranged in the interlayer space, and hydrolysis of the metal ions leads to the formation of isolated clusters in the surfactant crystals via limited condensation reactions. Immersing the surfactant crystals in formamide promotes nanosheet formation through the self-assembly of clusters by templating the morphologies of the crystals generated from surfactants crystals. Our approach opens a flatland in amorphous 2D world.

Achieving Exceptional Volumetric Desalination Capacity Using Compact MoS2 Nanolaminates

Capacitive deionization (CDI) has emerged as a promising technology for freshwater recovery from low-salinity brackish water. It is still inapplicable in specific scenarios (e.g., households, islands, or offshore platforms) due to too low volumetric adsorption capacities. In this study, a high-density semi-metallic molybdenum disulfide (1T'-MoS2) electrode with compact architecture obtained by restacking of exfoliated nanosheets, which achieve high capacitance up to ≈277.5 F cm-3 under an ultrahigh scan rate of 1000 mV s-1 with a lower charge-transfer resistance and nearly tenfold higher electrochemical active surface area than the 2H-MoS2 electrode, is reported. Furthermore, 1T'-MoS2 electrode demonstrates exceptional volumetric desalination capacity of 65.1 mgNaCl cm-3 in CDI experiments. Ex situ X-ray diffraction (XRD) reveal that the cation storage mechanism with the dynamic expansion of 1T'-MoS2 interlayer to accommodate cations such as Na+, K+, Ca2+, and Mg2+, which in turn enhances the capacity. Theoretical analysis unveils that 1T' phase is thermodynamically preferable over 2H phase, the ion hydration and channel confinement also play critical role in enhancing ion adsorption. Overall, this work provides a new method to design compact 2D-layered nanolaminates with high-volumetric performance for CDI desalination.

A library of 2D electronic material inks synthesized by liquid-metal-assisted intercalation of crystal powders

Solution-processable 2D semiconductor inks based on electrochemical molecular intercalation and exfoliation of bulk layered crystals using organic cations has offered an alternative pathway to low-cost fabrication of large-area flexible and wearable electronic devices. However, the growth of large-piece bulk crystals as starting material relies on costly and prolonged high-temperature process, representing a critical roadblock towards practical and large-scale applications. Here we report a general liquid-metal-assisted approach that enables the electrochemical molecular intercalation of low-cost and readily available crystal powders. The resulted solution-processable MoS2 nanosheets are of comparable quality to those exfoliated from bulk crystals. Furthermore, this method can create a rich library of functional 2D electronic inks ( >50 types), including 2D wide-bandgap semiconductors of low electrical conductivity. Lastly, we demonstrated the all-solution-processable integration of 2D semiconductors with 2D conductors and 2D dielectrics for the fabrication of large-area thin-film transistors and memristors at a greatly reduced cost.

Next-Generation Green Hydrogen: Progress and Perspective from Electricity, Catalyst to Electrolyte in Electrocatalytic Water Splitting

Green hydrogen from electrolysis of water has attracted widespread attention as a renewable power source. Among several hydrogen production methods, it has become the most promising technology. However, there is no large-scale renewable hydrogen production system currently that can compete with conventional fossil fuel hydrogen production. Renewable energy electrocatalytic water splitting is an ideal production technology with environmental cleanliness protection and good hydrogen purity, which meet the requirements of future development. This review summarizes and introduces the current status of hydrogen production by water splitting from three aspects: electricity, catalyst and electrolyte. In particular, the present situation and the latest progress of the key sources of power, catalytic materials and electrolyzers for electrocatalytic water splitting are introduced. Finally, the problems of hydrogen generation from electrolytic water splitting and directions of next-generation green hydrogen in the future are discussed and outlooked. It is expected that this review will have an important impact on the field of hydrogen production from water.

Tuning Vertical Electrodeposition for Dendrites-Free Zinc-Ion Batteries

Manipulating the crystallographic orientation of zinc deposition is recognized as an effective approach to address zinc dendrites and side reactions for aqueous zinc-ion batteries (ZIBs). We introduce 2-methylimidazole (Mlz) additive in zinc sulfate (ZSO) electrolyte to achieve vertical electrodeposition with preferential orientation of the (100) and (110) crystal planes. Significantly, the zinc anode exhibited long lifespan with 1500 h endurance at 1 mA cm-2 and an excellent 400 h capability at a depth of discharge (DOD) of 34% in Zn||Zn battery configurations, while in Zn||MnO2 battery assemblies, a capacity retention of 68.8% over 800 cycles is attained. Theoretical calculation reveals that the strong interactions between Mlz and (002) plane impeding its growth, while Zn atoms exhibit lower migration energy barrier and superior mobility on (100) and (110) crystal planes guaranteed the heightened mobility of zinc atoms on the (100) and (110) crystal planes, thus ensuring their superior ZIB performance than that with only ZSO electrolyte, which offers a route for designing next-generation high energy density ZIB devices.

Effect of replacing titanium with zirconium on the solubility of lithium in the LixZryTi1-yS2 system

Solid solutions of ZryTi1-yS2 (y = 0.1, 0.2) compositions have been synthesized. The electrochemical introduction of lithium is carried out by the galvanostatic intermittent titration technique. An isothermal section of the phase diagram of the LixZryTi1-yS2 system at room temperature has been constructed. The limiting solubility of lithium in solid solutions decreases with an increasing amount of titanium substituted for zirconium. It was noted that at low lithium concentrations (x < 1), there is no change in the E(x) dependence when titanium is replaced by zirconium. This may indicate the formation of separate TiS2 and ZrS2 layers.

Intercalation in 2D materials and in situ studies

Intercalation of atoms, ions and molecules is a powerful tool for altering or tuning the properties - interlayer interactions, in-plane bonding configurations, Fermi-level energies, electronic band structures and spin-orbit coupling - of 2D materials. Intercalation can induce property changes in materials related to photonics, electronics, optoelectronics, thermoelectricity, magnetism, catalysis and energy storage, unlocking or improving the potential of 2D materials in present and future applications. In situ imaging and spectroscopy technologies are used to visualize and trace intercalation processes. These techniques provide the opportunity for deciphering important and often elusive intercalation dynamics, chemomechanics and mechanisms, such as the intercalation pathways, reversibility, uniformity and speed. In this Review, we discuss intercalation in 2D materials, beginning with a brief introduction of the intercalation strategies, then we look into the atomic and intrinsic effects of intercalation, followed by an overview of their in situ studies, and finally provide our outlook.

Thermally Activated Photoluminescence Induced by Tunable Interlayer Interactions in Naturally Occurring van der Waals Superlattice SnS/TiS2

van der Waals (vdW) superlattices, comprising different 2D materials aligned alternately by weak interlayer interactions, offer versatile structures for the fabrication of novel semiconductor devices. Despite their potential, the precise control of optoelectronic properties with interlayer interactions remains challenging. Here, we investigate the discrepancies between the SnS/TiS2 superlattice (SnTiS3) and its subsystems by comprehensive characterization and DFT calculations. The disappearance of certain Raman modes suggests that the interactions alter the SnS subsystem structure. Specifically, such structural changes transform the band structure from indirect to direct band gap, causing a strong PL emission (∼2.18 eV) in SnTiS3. In addition, the modulation of the optoelectronic properties ultimately leads to the unique phenomenon of thermally activated photoluminescence. This phenomenon is attributed to the inhibition of charge transfer induced by tunable intralayer strains. Our findings extend the understanding of the mechanism of interlayer interactions in van der Waals superlattices and provide insights into the design of high-temperature optoelectronic devices.

Advances in Electrochemical Liquid-Phase Transmission Electron Microscopy for Visualizing Rechargeable Battery Reactions

This review presents an overview of the application of electrochemical liquid-phase transmission electron microscopy (ELP-TEM) in visualizing rechargeable battery reactions. The technique provides atomic-scale spatial resolution and real-time temporal resolution, enabling direct observation and analysis of battery materials and processes under realistic working conditions. The review highlights key findings and insights obtained by ELP-TEM on the electrochemical reaction mechanisms and discusses the current limitations and future prospects of ELP-TEM, including improvements in spatial and temporal resolution and the expansion of the scope of materials and systems that can be studied. Furthermore, the review underscores the critical role of ELP-TEM in understanding and optimizing the design and fabrication of high-performance, long-lasting rechargeable batteries.

Solution-Processable and Printable Two-Dimensional Transition Metal Dichalcogenide Inks

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with layered crystal structures have been attracting enormous research interest for their atomic thickness, mechanical flexibility, and excellent electronic/optoelectronic properties for applications in diverse technological areas. Solution-processable 2D TMD inks are promising for large-scale production of functional thin films at an affordable cost, using high-throughput solution-based processing techniques such as printing and roll-to-roll fabrications. This paper provides a comprehensive review of the chemical synthesis of solution-processable and printable 2D TMD ink materials and the subsequent assembly into thin films for diverse applications. We start with the chemical principles and protocols of various synthesis methods for 2D TMD nanosheet crystals in the solution phase. The solution-based techniques for depositing ink materials into solid-state thin films are discussed. Then, we review the applications of these solution-processable thin films in diverse technological areas including electronics, optoelectronics, and others. To conclude, a summary of the key scientific/technical challenges and future research opportunities of solution-processable TMD inks is provided.

Phase Engineering of Nanomaterials: Transition Metal Dichalcogenides

Crystal phase, a critical structural characteristic beyond the morphology, size, dimension, facet, etc., determines the physicochemical properties of nanomaterials. As a group of layered nanomaterials with polymorphs, transition metal dichalcogenides (TMDs) have attracted intensive research attention due to their phase-dependent properties. Therefore, great efforts have been devoted to the phase engineering of TMDs to synthesize TMDs with controlled phases, especially unconventional/metastable phases, for various applications in electronics, optoelectronics, catalysis, biomedicine, energy storage and conversion, and ferroelectrics. Considering the significant progress in the synthesis and applications of TMDs, we believe that a comprehensive review on the phase engineering of TMDs is critical to promote their fundamental studies and practical applications. This Review aims to provide a comprehensive introduction and discussion on the crystal structures, synthetic strategies, and phase-dependent properties and applications of TMDs. Finally, our perspectives on the challenges and opportunities in phase engineering of TMDs will also be discussed.

Metal telluride nanosheets by scalable solid lithiation and exfoliation

Transition metal tellurides (TMTs) have been ideal materials for exploring exotic properties in condensed-matter physics, chemistry and materials science1-3. Although TMT nanosheets have been produced by top-down exfoliation, their scale is below the gram level and requires a long processing time, restricting their effective application from laboratory to market4-8. We report the fast and scalable synthesis of a wide variety of MTe2 (M = Nb, Mo, W, Ta, Ti) nanosheets by the solid lithiation of bulk MTe2 within 10 min and their subsequent hydrolysis within seconds. Using NbTe2 as a representative, we produced more than a hundred grams (108 g) of NbTe2 nanosheets with 3.2 nm mean thickness, 6.2 µm mean lateral size and a high yield (>80%). Several interesting quantum phenomena, such as quantum oscillations and giant magnetoresistance, were observed that are generally restricted to highly crystalline MTe2 nanosheets. The TMT nanosheets also perform well as electrocatalysts for lithium-oxygen batteries and electrodes for microsupercapacitors (MSCs). Moreover, this synthesis method is efficient for preparing alloyed telluride, selenide and sulfide nanosheets. Our work opens new opportunities for the universal and scalable synthesis of TMT nanosheets for exploring new quantum phenomena, potential applications and commercialization.

Electrochemical exfoliation of 2D materials beyond graphene

After the discovery of graphene in 2004, the field of atomically thin crystals has exploded with the discovery of thousands of 2-dimensional materials (2DMs) with unique electronic and optical properties, by making them very attractive for a broad range of applications, from electronics to energy storage and harvesting, and from sensing to biomedical applications. In order to integrate 2DMs into practical applications, it is crucial to develop mass scalable techniques providing crystals of high quality and in large yield. Electrochemical exfoliation is one of the most promising methods for producing 2DMs, as it enables quick and large-scale production of solution processable nanosheets with a thickness well below 10 layers and lateral size above 1 μm. Originally, this technique was developed for the production of graphene; however, in the last few years, this approach has been successfully extended to other 2DMs, such as transition metal dichalcogenides, black phosphorous, hexagonal boron nitride, MXenes and many other emerging 2D materials. This review first provides an introduction to the fundamentals of electrochemical exfoliation and then it discusses the production of each class of 2DMs, by introducing their properties and giving examples of applications. Finally, a summary and perspective are given to address some of the challenges in this research area.

Scalable Synthesis of Bilayer Graphene at Ambient Temperature

In this work, we develop for the first time a facile chemical lithiation-assisted exfoliation approach to the controllable and scalable preparation of bilayer graphene. Biphenyl lithium (Bp-Li), a strong reducing reagent, is selected to realize the spontaneous Li-intercalation into graphite at ambient temperature, forming lithium graphite intercalation compounds (Li-GICs). The potential of Bp-Li (0.11 V vs Li/Li+), which is just lower than the potential of stage-2 lithium intercalation (0.125 V), enables the precise lithiation of graphite to stage-2 Li-GICs (LiC12). Intriguingly, the exfoliation of LiC12 leads to the bilayer-favored production of graphene, giving a high selectivity of 78%. Furthermore, the mild intercalation-exfoliation procedure yields high-quality graphene with negligible structural deterioration. The obtained graphene exhibits ultralow defect density (ID/IG ∼ 0.14) and a considerably high C/O ratio (∼29.7), superior to most current state-of-the-art techniques. This simple and scalable strategy promotes the understanding of chemical Li-intercalation methods for preparing high-quality graphene and shows great potential for layer-controlled engineering.

Tuning d-Orbital Electronic Structure via Au-Intercalated Two-Dimensional Fe3GeTe2 to Increase Surface Plasmon Activity

While extensive research has been dedicated to plasmon tuning within non-noble metals, prior investigations primarily concentrated on markedly augmenting the inherently low concentration of free carriers in materials with minimal consideration given to the influence of electron orbitals on surface plasmons. Here, we achieve successful intercalation of Au atoms into the layered structure of Fe3GeTe2 (FGT), thereby exerting control over the orbital electronic states or structure of FGT. This intervention not only amplifies the charge density and electron mobility but also mitigates the loss associated with interband transitions, resulting in increased two-dimensional FGT surface plasmon activity. As a consequence, Au-intercalated FGT detects crystal violet molecules as a surface-enhanced Raman scattering substrate, and the detection lines are 3 orders of magnitude higher than before Au intercalation. Our work provides insight for further studies on plasmon effects and the relation between surface plasmon resonance behavior and electronic structures.

Accelerating Sulfur Redox Kinetics by Electronic Modulation and Drifting Effects of Pre-Lithiation Electrocatalysts

Efficient catalyst design is crucial for addressing the sluggish multi-step sulfur redox reaction (SRR) in lithium-sulfur batteries (LiSBs), which are among the promising candidates for the next-generation high-energy-density storage systems. However, the limited understanding of the underlying catalytic kinetic mechanisms and the lack of precise control over catalyst structures pose challenges in designing highly efficient catalysts, which hinder the LiSBs' practical application. Here, drawing inspiration from the theoretical calculations, the concept of precisely controlled pre-lithiation SRR electrocatalysts is proposed. The dual roles of channel and surface lithium in pre-lithiated 1T'-MoS2 are revealed, referred to as the "electronic modulation effect" and "drifting effect", respectively, both of which contribute to accelerating the SRR kinetics. As a result, the thus-designed 1T'-Lix MoS2 /CS cathode obtained by epitaxial growth of pre-lithiated 1T'-MoS2 on cubic Co9 S8 exhibits impressive performance with a high initial specific capacity of 1049.8 mAh g-1 , excellent rate-capability, and remarkable long-term cycling stability with a decay rate of only 0.019% per cycle over 1000 cycles at 3 C. This work highlights the importance of precise control in pre-lithiation parameters and the synergistic effects of channel and surface lithium, providing new valuable insights into the design and optimization of SRR electrocatalysts for high-performance LiSBs.

Unleashing the potential of tungsten disulfide: Current trends in biosensing and nanomedicine applications

The discovery of graphene ignites a great deal of interest in the research and advancement of two-dimensional (2D) layered materials. Within it, semiconducting transition metal dichalcogenides (TMDCs) are highly regarded due to their exceptional electrical and optoelectronic properties. Tungsten disulfide (WS2) is a TMDC with intriguing properties, such as biocompatibility, tunable bandgap, and outstanding photoelectric characteristics. These features make it a potential candidate for chemical sensing, biosensing, and tumor therapy. Despite the numerous reviews on the synthesis and application of TMDCs in the biomedical field, no comprehensive study still summarizes and unifies the research trends of WS2 from synthesis to biomedical applications. Therefore, this review aims to present a complete and thorough analysis of the current research trends in WS2 across several biomedical domains, including biosensing and nanomedicine, covering antibacterial applications, tissue engineering, drug delivery, and anticancer treatments. Finally, this review also discusses the potential opportunities and obstacles associated with WS2 to deliver a new outlook for advancing its progress in biomedical research.

Recent Strategies for the Synthesis of Phase-Pure Ultrathin 1T/1T' Transition Metal Dichalcogenide Nanosheets

The past few decades have witnessed a notable increase in transition metal dichalcogenide (TMD) related research not only because of the large family of TMD candidates but also because of the various polytypes that arise from the monolayer configuration and layer stacking order. The peculiar physicochemical properties of TMD nanosheets enable an enormous range of applications from fundamental science to industrial technologies based on the preparation of high-quality TMDs. For polymorphic TMDs, the 1T/1T' phase is particularly intriguing because of the enriched density of states, and thus facilitates fruitful chemistry. Herein, we comprehensively discuss the most recent strategies for direct synthesis of phase-pure 1T/1T' TMD nanosheets such as mechanical exfoliation, chemical vapor deposition, wet chemical synthesis, atomic layer deposition, and more. We also review frequently adopted methods for phase engineering in TMD nanosheets ranging from chemical doping and alloying, to charge injection, and irradiation with optical or charged particle beams. Prior to the synthesis methods, we discuss the configuration of TMDs as well as the characterization tools mostly used in experiments. Finally, we discuss the current challenges and opportunities as well as emphasize the promising fields for the future development.

Ultrafast alternating-current exfoliation toward large-scale synthesis of graphene and its application for flexible supercapacitors

To facilitate the transition of laboratory research to industrial applications, it is critical to establish a reliable protocol for the mass synthesis of high-quality graphene. Here, we present an efficient electrochemical intercalation-based exfoliation approach utilizing alternating current that allows for the production of sub-kilogram quantities of graphene. This strategy involves repeatedly intercalating foreign anions and cations into the interlayer gaps of dual-graphite electrodes, accelerating the graphite expansion process and maximizing the exfoliation efficiency of both electrodes while inhibiting excessive anodic oxidation. The exfoliation process leads to high-yield graphene nanosheets (92 %, primarily 1-3 layers) with minimal structural deterioration (ID/IG ratio of 0.05), high purity (2.1 at% oxygen), and outstanding electrical property (7.28 × 104 S m-1). Notably, our scaled-up manufacturing technique produces a record-breaking throughput of 135 g h-1, improving on the best-reported exfoliation efficiency with direct current by 35%. Furthermore, the as-made graphene demonstrates a large reversible capacity of 102 mF cm-2 for flexible supercapacitors, with robust cyclability with 99.5% after 10,000 cycles, excellent mechanical flexibility, and exceptional serial integration for adjustable voltage output. The efficient and scalable method presents a significant advancement in the large-scale manufacture of graphene, with potential for widespread industrial applications.

Architecting a High-Energy-Density Rocking-Chair Zinc-Ion Batteries via Carbon-Wrapped Vanadium Dioxide

Aqueous zinc-ion batteries (ZIBs) show great potential in large-scale energy storage applications because of their low cost and high safety features, whereas the inefficient zinc utilization and uncontrollable dendrite issue of the zinc metal anode greatly limit their energy density and cycling stability. Herein, a carbon-wrapped vanadium dioxide (VO2@C) core-shell composite has been prepared and utilized as an intercalated anode of "rocking-chair" ZIBs. Benefiting from the carbon shell, the charge transfer and structural stability of VO2@C have been significantly improved, thus delivering a specific capacity of 425 mA h g-1 at 0.1 A g-1 and a capacity retention of 94.9% after 3000 cycles at 5 A g-1, better than that of VO2 (338 mA h g-1 and 59.2%). Further, the low Zn2+ intercalated potential (0.54 V vs Zn2+/Zn) and reversible Zn2+ intercalation/deintercalation behavior of VO2@C enable the successful construction of VO2@C||ZnMn2O4 "rocking-chair" ZIBs, which achieve a capacity of 104 mA h g-1 at 0.1 A g-1 and an exceptional energy density of 96.3 W h kg-1 at 74.1 W kg-1 (based on the total weight). This research enriches the currently available options for constructing high-energy-density energy storage systems.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Elucidating the Magnetoelastic Coupling, Pressure-Dependent Magnetic Behavior, and Anomalous Hall Effect in FexTi2S4 Intercalation Sulfides

Transition-metal chalcogenides with intercalated layered structures are interesting systems in material physics due to their attractive electronic and magnetic properties, with applications in the fields of magnetic refrigerators, catalysts, and thermoelectrics, among others. In this work, we studied in detail the structural, electronic, and magnetic properties of (Fe,Ti)-based sulfides with formula FexTi2S4 (x = 0.24, 0.32, and 0.42), prepared as polycrystalline materials under high-pressure conditions. They present a layered Heideite-type crystal structure, as assessed by synchrotron X-ray diffraction. A local structure analysis using Fe K-edge extended X-ray-absorption fine structure (EXAFS) data unveiled a conspicuous contraction of the main Fe-S bond in Fe0.24Ti2S4 at the vicinity of the magnetic transition 60-80 K. We suggest that this anomaly is related to magnetoelastic coupling effects. The EXAFS analysis allowed extraction of the Einstein temperatures (θE), i.e., the phonon contribution to the specific heat, for the two bond pairs Fe-S(1) [θE ≈318 K; 290 K (C/T)] and Fe-Ti(1) [θE ≈218 K; 190 K (C/T)]. In addition to the structural and local vibrational measurements, we probed the magnetic properties using magneto-calorimetry, magnetometry under applied pressure, magnetoresistance (MR), and Hall effect measurements. We observed the appearance of a broad peak in the specific heat around 120 K in the x = 0.42 compound that we associated with an antiferromagnetic ordering electronic transition. We found that the antiferromagnetic transition temperature is pressure and composition sensitive and reduces at 1.2 GPa by ∼12 and ∼3 K, for the members with x = 0.24 and x = 0.42, respectively. Similarly, the saturation magnetization in the ordered phase depends on both pressure and iron content, reducing its value by 50, 90, and 30% for x = 0.24, 0.32, and 0.42, respectively. We observed clear jumps in the magnetic hysteresis loops, MR, and anomalous Hall effect (AHE) below 2 K at fields around 2-4 T. We associated this observation with the metamagnetic transitions; from the Berry-curvature a decoupling parameter of SH = 0.12 V-1 is determined. Comparison of the results on the temperature-dependent magnetization, MR, and AHE elucidates a strong inelastic scattering contribution to the AHE at higher temperatures due to the cluster spin-glass phase.

Simultaneous Electrochemical Exfoliation and Functionalization of 2H-MoS2 for Supercapacitor Electrodes

MoS2 is a promising semiconducting material that has been widely studied for applications in catalysis and energy storage. The covalent chemical functionalization of MoS2 can be used to tune the optoelectronic and chemical properties of MoS2 for different applications. However, 2H-MoS2 is typically chemically inert and difficult to functionalize directly and thus requires pretreatments such as a phase transition to 1T-MoS2 or argon plasma bombardment to introduce reactive defects. Apart from being inefficient and inconvenient, these methods can cause degradation of the desirable properties and introduce unwanted defects. Here, we demonstrate that 2H-MoS2 can be simultaneously electrochemically exfoliated and chemically functionalized in a facile and scalable procedure to fabricate functionalized thin (∼4 nm) MoS2 layers. The aryl diazonium salts used for functionalization have not only been successfully covalently grafted onto the 2H-MoS2, as verified by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, but also aid the exfoliation process by increasing the interlayer spacing and preventing restacking. Electrochemical energy storage is one application area to which this material is particularly suited, and characterization of supercapacitor electrodes using this exfoliated and functionalized material revealed that the specific capacitance was increased by ∼25% when functionalized. The methodology demonstrated for the simultaneous production and functionalization of two-dimensional (2D) materials is significant, as it allows for control over the flake morphology with increased repeatability. This electrochemical functionalization technique could also be extended to other types of transition-metal dichalcogenides (TMDs), which are also typically chemically inert with different functional species to adjust to specific applications.

Green Colloidal Synthesis of MoS2 Nanoflakes

Currently, two approaches dominate the large-scale production of MoS2: liquid-phase exfoliation, referred to as the top-down approach, and bottom-up colloidal synthesis from molecular precursors. Known colloidal synthesis approaches utilize toxic precursors. Here, an alternative green route for the bottom-up synthesis of MoS2 nanoflakes (NFs) is described. The NFs were synthesized by colloidal synthesis using [Mo(CH3COO)2]2 and a series of sulfur (S)-precursors including thioacetamide (TAA), 3-mercaptopropionic acid (3-MPA), l-cysteine (L-CYS), mercaptosuccinic acid (MSA), 11-mercaptoundecanoic acid (MUA), 1-dodecanethiol (DDTH), and di-tert-butyl disulfide (DTBD). While TAA, an S-precursor most commonly used for MoS2 NF preparation, is a known carcinogen, the other investigated S-precursors have low or no known toxicity. High-resolution scanning transmission electron microscopy (HR-STEM) and grazing incidence wide-angle X-ray scattering (GIWAXS) confirmed that in all cases, the syntheses yielded single-layer MoS2 NFs with lateral sizes smaller than 15 nm and a well-defined crystal structure. Electronic absorption and Raman spectra showed characteristic features associated with the MoS2 monolayers. The evolution of the absorption spectra of the growth solution during the syntheses reveals how the kinetics of the NF formation is affected by the S-precursor as well as the nature of the coordinating ligands.

Highly Reversible Zn Metal Anode with Low Voltage Hysteresis Enabled by Tannic Acid Chemistry

The zinc dendrites and side reactions formed on the zinc anode have greatly hindered the development of aqueous zinc-ion batteries (ZIBs). Herein, we introduce tannic acid (TA) as an additive in the ZnSO4 (ZSO) electrolyte to enhance the reversible Zn plating/stripping behavior. TA molecules are found to adsorb onto the zinc surface, forming a passivation layer and replacing some of the H2O molecules in the Zn2+ solvation sheath to form the [Zn(H2O)6-xTAx]2+ complex; this process effectively prevents side reactions. Moreover, the lower desolvation energy barrier of the [Zn(H2O)6-xTAx]2+ structure facilitates uniform Zn metal deposition and enables a stable plating/stripping lifespan of 2500 h with low voltage hysteresis (53 mV at 0.5 mA cm-2) as compared to the ZSO electrolyte (167 h and 104 mV). Additionally, the incorporation of the MnO2 cathode in the TA + ZSO electrolyte shows improved cycling capacity retention, from 64% (ZSO) to 85% (TA + ZSO), after 250 cycles at 1 A g-1, demonstrating the effectiveness of the TA additive in enhancing the performance of ZIBs.

Design principles for 2D transition metal dichalcogenides toward lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are regarded as a promising candidate for next-generation energy storage systems owing to their remarkable energy density, resource availability, and environmental benignity. Nevertheless, severe shuttling effect, sluggish redox kinetics, large volumetric expansion, and uncontrollable dendrite growth hamper the practical applications. To address these intractable issues, two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged expeditiously as an essential material strategy. Herein, this review emphasizes the development and application of 2D TMDs in Li-S batteries. It starts with introducing the fundamentals of Li-S batteries and common synthetic routes of TMDs, followed by summarizing the employment of pristine, hybrid, and defective TMDs in the realm of expediting sulfur chemistry and stabilizing lithium anode. Finally, the development roadmap and possible research directions of TMDs are proposed to offer guidance for the future design of high-performance Li-S batteries.

Out-of-plane polarization modulated band alignments inβ-In2X3/α-In2X3(X = S and Se) vdW heterostructures

The construction of two-dimensional (2D) van der Waals (vdW) heterostructures is an effective strategy to overcome the intrinsic disadvantages of individual 2D materials. Herein, by employing first-principles calculations, the electronic structures and potential applications in the photovoltaic field of theβ-In2X3/α-In2X3(X = S and Se) vdW heterostructures have been systematically unraveled. Interestingly, the band alignments ofβ-In2S3/α-In2S3,β-In2Se3/α-In2Se3, andβ-In2Se3/α-In2S3heterostructures can be transformed from type-I to type-II by switching the polarization direction ofα-In2X3layers. It is highlighted that the light-harvesting ability of theβ-In2X3/α-In2X3vdW heterostructures is significantly higher than the corresponding monolayers in nearly the entire visible light region. Interestingly, type-IIβ-In2S3/α-In2Se3↓ heterostructure can achieve the power conversion efficiency of 17.9%, where theα-In2Se3layer acts as a donor and theβ-In2S3layer displays as the acceptor. The present research not only provides an in-depth understanding that the out-of-plane polarization ofα-In2X3monolayers can efficiently modulate the band edge alignment of theβ-In2X3/α-In2X3vdW heterostructures, but also paves the way for the application of these heterostructures in the field of photovoltaics and optoelectronics.

Novel Fabrication of Basalt Nanosheets with Ultrahigh Aspect Ratios Toward Enhanced Mechanical and Dielectric Properties of Aramid Nanofiber-Based Composite Nanopapers

The rapid development of modern electrical equipment has led to urgent demands for electrical insulating materials with mechanical reliability and excellent dielectric properties. Herein, basalt nanosheets (BSNs) with high aspect ratios (≈780.1) are first exfoliated from basalt scales (BS) through a reliable chemical/mechanical approach. Meanwhile, inspired by the layered architecture of natural nacre, nacre-mimetic composite nanopapers are reported containing a 3D aramid nanofibers (ANF) framework as a matrix and BSNs as ideal building blocks through vacuum-assisted filtration. The as-prepared ANF-BSNs composite nanopapers exhibit considerably enhanced mechanical properties with ultralow BSNs content. These superiorities are wonderfully integrated with exceptional dielectric breakdown strength, prominent volume resistivity, and extremely low dielectric constant and loss, which are far superior to conventional nacre-mimetic composite nanopapers. Notably, the tensile strength and breakdown strength of ANF-BSNs composite nanopapers with a mere 1.0 wt% BSNs reach 269.40 MPa and 77.91 kV mm-1 , respectively, representing an 87% and 133% increase compared to those of the control ANF nanopaper. Their properties are superior to those of previously reported nacre-mimetic composite nanopapers and commercial insulating micropapers, indicating that ANF-BSNs composite nanopapers are a highly promising electrical insulating material for miniaturized high-power electrical equipment.

Electrochemical molecular intercalation and exfoliation of solution-processable two-dimensional crystals

Electrochemical molecular intercalation of layered semiconducting crystals with organic cations followed by ultrasonic exfoliation has proven to be an effective approach to producing a rich family of organic/inorganic hybrid superlattices and high-quality, solution-processable 2D semiconductors. A traditional method for exfoliating 2D crystals relies on the intercalation of inorganic alkali metal cations. The organic cations (e.g., alkyl chain-substituted quaternary ammonium cations) are much larger than their inorganic counterparts, and the bulky molecular structure endows distinct intercalation and exfoliation chemistry, as well as molecular tunability. By using this protocol, many layered 2D crystals (including graphene, black phosphorus and versatile metal chalcogenides) can be electrochemically intercalated with organic quaternary alkylammonium cations. Subsequent solution-phase exfoliation of the intercalated compounds is realized by regular bath sonication for a short period (5-30 min) to produce free-standing, thin 2D nanosheets. It is also possible to graft additional ligands on the nanosheet surface. The thickness of the exfoliated nanosheets can be measured by using atomic force microscopy and Raman spectroscopy. Modifying the chemical structure and geometrical configuration of alkylammonium cations results in different exfoliation behavior and a family of versatile organic/inorganic hybrid superlattices with tunable physical/chemical properties. The whole protocol takes ~6 h for the successful production of stable, ultrathin 2D nanosheet dispersion in solution and another 11 h for depositing thin films and transferring them onto an arbitrary surface. This protocol does not require expertise beyond basic electrochemistry knowledge and conventional colloidal nanocrystal synthesis and processing.

Crystal-Phase Engineering of Two-Dimensional Transition-Metal Dichalcogenides for Surface-Enhanced Raman Scattering: A Perspective

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have emerged as promising materials for surface-enhanced Raman scattering (SERS) due to their unique electronic, optical, and mechanical properties. In this Perspective, we briefly introduce the fundamental properties, crystal-phase configurations, and phase transition strategies of TMDs materials. We then discuss the importance of the crystal phase in determining the SERS effect of TMDs, highlighting recent advances in phase-engineering approaches to affording remarkable SERS performance. By considering the current challenges and future directions for improving the crystal-phase engineering of TMDs in SERS, we also offer new insights into the design and synthesis of more promising TMD-based SERS substrates.

Antioxidative 2D Bismuth Selenide via Halide Passivation for Enhanced Device Stability

The topological insulator 2D Bi₂Se₃ is promising for electronic devices due to its unique electronic properties; however, it is challenging to prepare antioxidative nanosheets since Bi₂Se₃ is prone to oxidation. Surface passivation using ligand agents after Bi₂Se₃ exfoliation works well to protect the surface, but the process is time-consuming and technically challenging; a passivation agent that is stable under a highly biased potential is significant for in situ passivation of the Bi₂Se₃ surface. In this work, the roles of halide anions (Cl^-, Br^-, and I^-) in respect of the chemical properties of synthetic Bi₂Se₃ nanosheets during electrochemical intercalated exfoliation were investigated to determine the antioxidation capacity. It was found that Bi₂Se₃ nanosheets prepared in a solution of tetrabutylammonium chloride (TBA⁺ and Cl^-) have the best oxidation resistance via the surface bonding of Bi with Cl, which promotes obtaining better device stability. This work paves an avenue for adjusting the components of the electrolyte to further promote the stability of 2D Bi₂Se₃-nanosheet-based electronic devices.

Regulating the Electrical and Mechanical Properties of TaS2 Films via van der Waals and Electrostatic Interaction for High Performance Electromagnetic Interference Shielding

Low-dimensional transition metal dichalcogenides (TMDs) have unique electronic structure, vibration modes, and physicochemical properties, making them suitable for fundamental studies and cutting-edge applications such as silicon electronics, optoelectronics, and bioelectronics. However, the brittleness, low toughness, and poor mechanical and electrical stabilities of TMD-based films limit their application. Herein, a TaS2 freestanding film with ultralow void ratio of 6.01% is restacked under the effect of bond-free van der Waals (vdW) interactions within the staggered 2H-TaS2 nanosheets. The restacked films demonstrated an exceptionally high electrical conductivity of 2,666 S cm-1, electromagnetic interference shielding effectiveness (EMI SE) of 41.8 dB, and absolute EMI SE (SSE/t) of 27,859 dB cm2 g-1, which is the highest value reported for TMD-based materials. The bond-free vdW interactions between the adjacent 2H-TaS2 nanosheets provide a natural interfacial strain relaxation, achieving excellent flexibility without rupture after 1,000 bends. In addition, the TaS2 nanosheets are further combined with the polymer fibers of bacterial cellulose and aramid nanofibers via electrostatic interactions to significantly enhance the tensile strength and flexibility of the films while maintaining their high electrical conductivity and EMI SE.This work provides promising alternatives for conventional materials used in EMI shielding and nanodevices.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

2D Transition Metal Dichalcogenides for Photocatalysis

Two-dimensional (2D) transition metal dichalcogenides (TMDs), a rising star in the post-graphene era, are fundamentally and technologically intriguing for photocatalysis. Their extraordinary electronic, optical, and chemical properties endow them as promising materials for effectively harvesting light and catalyzing the redox reaction in photocatalysis. Here, we present a tutorial-style review of the field of 2D TMDs for photocatalysis to educate researchers (especially the new-comers), which begins with a brief introduction of the fundamentals of 2D TMDs and photocatalysis along with the synthesis of this type of material, then look deeply into the merits of 2D TMDs as co-catalysts and active photocatalysts, followed by an overview of the challenges and corresponding strategies of 2D TMDs for photocatalysis, and finally look ahead this topic.

Controllable Synthesis of 2D Materials by Electrochemical Exfoliation for Energy Storage and Conversion Application

2D materials have captured much recent research interest in a broad range of areas, including electronics, biology, sensors, energy storage, and others. In particular, preparing 2D nanosheets with high quality and high yield is crucial for the important applications in energy storage and conversion. Compared with other prevailing synthetic strategies, the electrochemical exfoliation of layered starting materials is regarded as one of the most promising and convenient methods for the large-scale production of uniform 2D nanosheets. Here, recent developments in electrochemical delamination are reviewed, including protocols, categories, principles, and operating conditions. State-of-the-art methods for obtaining 2D materials with small numbers of layers-including graphene, black phosphorene, transition metal dichalcogenides and MXene-are also summarized and discussed in detail. The applications of electrochemically exfoliated 2D materials in energy storage and conversion are systematically reviewed. Drawing upon current progress, perspectives on emerging trends, existing challenges, and future research directions of electrochemical delamination are also offered.

Fabrication of liquid cell for in situ transmission electron microscopy of electrochemical processes

Fundamentally understanding the complex electrochemical reactions that are associated with energy devices (e.g., rechargeable batteries, fuel cells and electrolyzers) has attracted worldwide attention. In situ liquid cell transmission electron microscopy (TEM) offers opportunities to directly observe and analyze in-liquid specimens without the need for freezing or drying, which opens up a door for visualizing these complex electrochemical reactions at the nano scale in real time. The key to the success of this technique lies in the design and fabrication of electrochemical liquid cells with thin but strong imaging windows. This protocol describes the detailed procedures of our established technique for the fabrication of such electrochemical liquid cells (~110 h). In addition, the protocol for the in situ TEM observation of electrochemical reactions by using the nanofabricated electrochemical liquid cell is also presented (2 h). We also show and analyze experimental results relating to the electrochemical reactions captured. We believe that this protocol will shed light on strategies for fabricating high-quality TEM liquid cells for probing dynamic electrochemical reactions in high resolution, providing a powerful research tool. This protocol requires access to a clean room equipped with specialized nanofabrication setups as well as TEM characterization equipment.

Boosting Electrocatalytic Reduction of CO2 to HCOOH on Ni Single Atom Anchored WTe2 Monolayer

Achieving efficient conversion of carbon dioxide (CO₂ ) to formic acid (HCOOH) at mild conditions is a promising means to reduce greenhouse gas emission and mitigate the energy crisis. Herein, spin-polarized density functional theory calculations with van der Waals corrections (DFT+D3) are performed to analyze the catalytic activity of seven metals (Ti, Fe, Ni, Cu, Zn, In, and Sn) anchored on a tungsten ditelluride monolayer (M@WTe₂ ) and screen favorable CO₂ reduction pathways. These results demonstrate that Ni single atoms strongly bind to the WTe₂ monolayer and exist in isolated form due to the high diffusion barriers. Also, Ni-anchored WTe₂ monolayer (Ni@WTe₂ ) possesses a considerably low limiting-potential (-0.11 V vs reversible hydrogen electrode) to convert CO₂ to HCOOH due to moderate OCHO adsorption energy and a suppressed competing hydrogen evolution reaction (HER). Therefore, Ni@WTe₂ monolayer is a promising electrocatalytic material for the CO₂ reduction reaction (CO₂ RR). This study sheds light on strategies of designing single metal atom anchored WTe₂ catalysts for improved CO₂ RR performances.

Hierarchical van der Waals Heterostructure Strategy to Form Stable Transition Metal Dichalcogenide Dispersions

Although various methods have been developed to disperse transition metal dichalcogenides (TMDCs) in aqueous environments, the methodology to generate stable TMDC dispersions remains challenging. Here, we developed a hierarchical van der Waals (vdW) heterostructure-based strategy to disperse few-layered TMDCs (WS2, MoS2, WSe2, and MoSe2) using both hexagonal boron nitride (hBN) and sodium cholate (SC) as synergistic vdW surfactants. By showing long-term stability of up to 3 years, the extinction spectra of these TMDC/hBN/SC dispersions exhibit the most blue-shifted excitonic transitions, low background extinction, good colloidal stability, and dispersion stability upon ultracentrifugation compared to other dispersion methods. Hierarchical stacking having TMDCs and hBN/SC as core and shell parts is probed by electrostatic/atomic force microscopy and zeta potential, and its origin was attributed to surface energy matches. Along with the synergetic effect between TMDCs and hBN, the blue shift was ascribed to compressive strain on the TMDCs caused by hBN wrapping. The results of transmission electron microscopy show that the TMDCs in the dispersions have defective, few-layered structures with flake sizes that are less than a few hundred nm2. Raman spectroscopy is used to study not only the existence of compressive strain but also various interlayer coupling between TMDC and hBN. The hierarchical structures of TMDC/hBN/SC are discussed in terms of surface energies and topographies. This method is invaluable to provide a general methodology to disperse various surface-corrugated dimensional materials for various dispersion-based applications.

Deciphering the Space Charge Effect of the p-n Junction between Copper Sulfides and Molybdenum Selenides for Efficient Water Electrolysis in a Wide pH Range

Space charge transfer is crucial for an efficient electrocatalytic process, especially for narrow-band-gap metal sulfides/selenides. Herein, we designed and synthesized a core-shell structure which is an ultrathin MoSe₂ nanosheet coated CuS hollow nanoboxes (CuS@MoSe₂) to form an open p-n junction structure. The space charge effect in the p-n junction region will greatly improve electron mass transfer and conduction, and also have abundant active interfaces. It was used as a bifunctional electrocatalyst for water oxidation at a wide pH range. It exhibits a low overpotential of 49 mV for the HER and 236 mV for the OER at a current density of 10 mA·cm^-2 in acidic pH, 72 mV for the HER and 219 mV at 10 mA·cm^-2 for the OER in alkaline pH, and 62 mV for the HER and 230 mV at 10 mA·cm^-2 for the OER under neutral conditions. The experimental results and density functional theory calculations testify that the p-n junction in CuS@MoSe₂ designed and synthesized has a strong space charge region with a synergistic effect. The built-in field can boost the electron transport during the electrocatalytic process and can stabilize the charged active center of the p-n junction. This will be beneficial to improve the electrocatalytic performance. This work provides the understanding of semiconductor heterojunction applications and regulating the electronic structure of active sites.